1. Field of the Invention
The present invention relates generally to communications, and, more particularly, to a method and apparatus for frequency hopping in a spread spectrum digital cordless telephone system.
2. Description of the Related Art
The telecommunications industry has undergone explosive growth over the past several years. A significant contribution to this growth has been the high demand for radio communication services, such as cordless telephone service. Cordless telephones provide greater flexibility than traditional landline phones by permitting a user to move freely and not be physically restricted by a telephone cord. Although other types of radio telephones exist on the market (e.g., cellular, PCS, etc.), cordless telephones are a much cheaper alternative for providing telephone service within a limited operating range, such as a home, for example. Accordingly, cordless phones account for a significant portion of the vast radio telecommunications market.
A cordless telephone consists of a base unit and a remote unit that communicate over a radio frequency (RF) link. Generally, the remote unit takes the form of a cordless handset. The base unit couples to the public switched telephone network (PSTN) to provide telephonic services to a user of the remote unit via the RF link.
When the cordless telephone was initially introduced, the frequency of the radio channel used between the base and remote units was typically constant. The use of a single radio channel between the base and remote units was not desirable, however, because it made the radio link more prone to interference by other radio systems. Additionally, using a single radio channel also made eavesdropping on telephone conversations relatively easy to accomplish.
To attempt to alleviate these problems, some cordless phones were equipped such that the user could manually change the radio channel between the base and remote units, which was usually accomplished by a “channel change” button on the remote unit. These phones, however, were not very successful because the user was inconvenienced by having to manually change the radio channel. And, when the channel was changed, there typically was not enough variation in frequency between the limited number of radio channels to completely eliminate the radio interference. Furthermore, the limited number of channels used by these phones also did not significantly reduce the possibility of eavesdropping.
When the frequency-hopped spread spectrum (FHSS) cordless phone was introduced, it significantly reduced the aforementioned problems of radio interference and eavesdropping commonly associated with the previous model cordless phones. The base and remote unit of an FHSS cordless phone synchronously hop across a pseudorandomly-ordered series of frequencies, which greatly improves the clarity, range, and security of communication between the base and remote units.
Currently, however, FHSS cordless phones use “dwell” time (i.e., time spent on a given frequency) to change the base and remote unit's radio transceivers to a new frequency of the pseudorandomly-ordered frequency pattern. Because the carrier frequencies are unstable as they are changed, no information can be transmitted or received during this dwell time. To accommodate an effective throughput data rate of 32 kbits/sec, which is required for full-duplex digitized voice transmission, the voice data transmitted between the base and remote units must be queued and transmitted at a much higher rate over the radio link. This higher data rate results in a decreased signal-to-noise ratio, which diminishes the sensitivity and range of the radio link between the remote and base units of the FHSS phone.
The present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.